1. Field of the Invention
The present invention relates to a spread spectrum (SS) communication technique, and more particularly, to a spread spectrum communication system, a clear channel assessment (CCA) device utilizing a plurality of receiving paths for clear channel assessment, and a related method.
2. Description of the Prior Art
Spread spectrum is a communication technique of spreading the spectrum of a transmitted signal into a frequency band having a wider bandwidth, and is commonly used in wireless communication. Spread spectrum technique can be more stringently divided into two categories: the first being that, after spread spectrum modulation is performed, the signal transmission bandwidth is much larger than the original signal bandwidth; the second being that a transmitting end will employ a unique symbol, which does not affect transmitted data, and a receiving terminal also has to utilize the unique symbol to decode the spread spectrum to acquire the actual data transmitted from the transmitting end. There are also two commonly used spread spectrum methods: direct-sequence spread spectrum (DSSS) and frequency-hopping spread spectrum (FHSS). Spread spectrum communication technique is immune to background noise, interference and self multi-path interference, and influence from electromagnetic interference (EMI) is also minimal. Therefore, through the use of the spread spectrum technique, code division multiple access (CDMA) communication can be realized, which allows more users to independently utilize wider bandwidths at the same time.
To achieve better receiving quality, a receiving end of a spread spectrum communication system utilizes a rake receiver to receive signal power via different wireless communication paths; however, the rake receiver has to start operation after finishing channel assessment, and is of little help for signal processing (e.g., packet detection) before channel assessment. Since packet assessment is the most significant part in signal reception, if packets cannot be detected correctly, subsequent signal detection and processing operations also cannot proceed properly.
Please refer to FIG. 1, which is a diagram of a conventional spread spectrum communication system 100. The spread spectrum communication system 100 includes an antenna 110, a radio frequency demodulator 115, an analog-to-digital converter (ADC) 120, a digital finite impulse response (DFIR) filter, a clear channel assessment (CCA) device 140, a switch circuit 150, a rake receiver 160 and a symbol examination circuit 170. The antenna 110 is for receiving a radio frequency (RF) signal SRF. The radio frequency demodulator 115 is coupled to the antenna 110, and is for demodulating the radio frequency signal SRF into an analog baseband signal SBB. The ADC 120 is coupled to the radio frequency demodulator 115, and is for converting the analog baseband signal SBB into a digital signal Sdig. The DFIR filter 130 is coupled to the ADC 120, and is for filtering the digital signal Sdig to generate a filtered signal Sf. The CCA device 140 is coupled to the DFIR filter 130, and is for generating a channel assessment signal Scca according to signal characteristics of the filtered signal Sf.
The switch circuit 150 is coupled to the CCA device 140, and is for referring to the channel assessment signal Scca to be selectively conducting (an “on” status) or non-conducting (an “off” status). The rake receiver 160 is coupled to the switch circuit 150, and is for receiving the filtered signal Sf to generate a received signal Sr when the switching circuit 150 is conducting. The symbol examination circuit 170 is coupled to the rake receiver 160, and is for performing symbol examination for the received signal Sr.
As shown in FIG. 1, the CCA device 140 includes a de-spreading processor 141, a switch 143, a first power calculator 145, a second power calculator 147 and a comparator 149. The de-spreading processor 141 is for receiving the filtered signal Sf and performing a de-spreading process for the filtered signal Sf to generate a de-spreading signal Sds accordingly. The switch 143 is coupled to the de-spreading processor 141, and refers to the de-spreading signal Sds to be selectively conducting or non-conducting. The first power calculator 145 is coupled to the switch 143, and is for receiving the de-spreading signal Sds, and calculating the power of the de-spreading signal Sds to generate a first calculated value C1. The second power calculator 147 is for receiving the filtered signal Sf, and calculating the power of the filtered signal Sf to generate a second calculated value C2. The comparator 149 is coupled to the first power calculator 145 and the second power calculator 147, and is for generating the channel assessment signal Scca according to the first calculated value C1 and the second calculated value C2.
The CCA device 140 can detect if there is a packet being transmitted by referring to signal characteristics of the filtered signal Sf. When a signal power after de-spreading (e.g., the calculated value C1) is larger than a certain percentage of a signal power before de-spreading, the CCA device 140 determines that a packet is detected. At this moment, the CCA device 140 will control the switch circuit 150 to be conducting via the channel assessment signal Scca, and activate the following rake receiver 160 and symbol examination circuit 170. Generally speaking, the rake receiver 160 can enhance signal detection quality; however, if the CCA device 140 cannot successfully detect a packet in time, the rake receiver 160 is unable to proceed to following operations.